Induced nuclear reactions: processes – systems – and elements – Fission reactor material treatment – Impurity removal
Patent
1996-03-26
1998-11-17
Bocure, Tesfaldet
Induced nuclear reactions: processes, systems, and elements
Fission reactor material treatment
Impurity removal
375326, 375341, 375340, H04L 2706
Patent
active
058387340
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to receiving equipment for digital transmissions of the orthogonal frequency division multiplex type and in particular to receiving equipment which can compensate for phase errors in the received digital signals.
BACKGROUND OF THE INVENTION
Orthogonal frequency division multiplex (OFDM) is a modulation technique which has been proprosed by the Eureka 147 digital broadcasting consortium. In such transmissions digital data is divided between a large number of adjacent carriers so that a relatively small amount of data is. carried on each carrier. This is the frequency division multiplex part of OFDM. The orthogonal part of the OFDM name arises because adjacent carriers are arranged to be mathematically orthogonal so that their sidebands may overlap but signals can still be received without adjacent carrier interference.
Digital data is modulated onto a carrier using quadrature phase shift keying (QPSK) or a higher level of quadrature amplitude modulation (QAM) such a 64 QAM or 256 QAM. FIG. 1 shows a phase diagram for a QPSK modulation scheme. The scheme provides four phase states which are represented by vectors, one in each quadrant of the phase diagram. Thus with a QPSK scheme a two bit word can be modulated onto a carrier by varying the phase of the carrier.
FIG. 2 shows a 16 QAM modulation scheme which provides 16 phase states, for each quadrant. The four vectors in the upper right quadrant are indicated on the figure. This scheme enables four bit words to be modulated onto a carrier by varying the phase and amplitude of the carrier.
The QPSE modulation scheme of FIG. 1 has a tolerance of 45.degree. to phase shift errors for the carrier. It will be appreciated that for the 16 QAM scheme of FIG. 2 this tolerance is reduced and for higher order QAM schemes e.g. 64 QAM, the phase shift error tolerance is reduced still further. Thus the minimization of phase shift errors in transmitter and receiver becomes important.
In a practical OFDM transmitter and receiver the need for modulators, filters, and demodulators for each carrier is avoided by use of the fast fourier transform (FFT) algorithmn to perform the modulation/de-modulation process on the many carriers. In order to transmit the many carriers the wide band frequency domain digital signal is transformed using an FFT into the time domain. This signal is then transmitted. In a receiver the reverse process is applied to produce the plurality of carriers. The FFT for a sample of the signal is known as a symbol and this is what is transmitted and then received.
There are various sources of phase noise in the transmission and reception of the signal and some of these are discussed below.
Phase errors due to thermally generated random noise affect the amplitudes and phases of the carriers in a way such that there is no relationship between the errors of different carriers in the same FFT frame or between the errors on a chosen carrier between different FFT frames.
Phase errors due to local oscillator phase noise in a receiver appear equally on all carriers within one FFT frame, but the value of this error is random in terms of its value for any or all carriers between one FFT frame and the next. The amplitudes of the carriers will not be affected by local oscillator phase noise.
A frequency error on the local oscillator can be interpreted as a phase error which is equal an all carriers in any one FFT frame and where the angle of such error progresses systematically from frame to frame at a rate dependent on the frequency error. It can be detected by detecting the phase error on any chosen carrier in every frame and calculating the average of the rate of progression. Using the average sill eliminate the effects of random noise.
An error in the timing of the FFT frame is equivalent to a uniform group delay error across the frequency band occupied by the carriers. Each carrier has a phase error which is directly related to its frequency and the delay value. The advance (or retard). of phase with carrier frequency
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H. Nakamura, et al "Power Efficient High-Level Modulation for High-Capacity Digital Radio Systems", The Transactions of the Institute of Electronics, Information and Comm. Engineers, E72, 1989, No. 5, Toyko, Japan, pp. 633-639.
F. Daffara, et al "Maximum Likelihood Frequency Detectors for Orthogonal Multicarrier Systems", IEEE International Conference on Communications, '93, May 1993, Conference Record, vol. 2/3, pp. 766-771.
Bocure Tesfaldet
British Broadcasting Corporation
Neuner George W.
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